1. Field of the Invention
The present invention relates to apparatus and methods for use with heating, ventilation, and air conditioning (HVAC) units and particularly to the use of fan-powered mixing box assemblies (also called power induction units (PIU) in variable air volume (VAV) and variable volume/variable temperature (VVT) systems.
2. Prior Art
The present invention relates to fan-powered mixing box assemblies and microcomputer controls used therewith.
In many large buildings, the economics of using several heating, ventilation and air conditioning (HVAC) units becomes impractical and large single HVAC units are used. In the usual VAV system application, a single large HVAC unit supplies conditioned air to the various zones via zone dampers, such zone dampers controlling air flow only. As various zone dampers modulate closed because the zone demands have been met, as established by a local thermostat, the air flow through the remaining dampers will increase. Methods of accommodating the additional and often excessive air flow that can result consist generally of air control systems such as discharge dampers and variable speed fans. Air flow sensors and air pressure sensors can be utilized in conjunction with the local thermostat controlling the zone damper. This capability allows the air flow to a given zone to be controlled independently of system pressure variations. The use of fan-powered mixing box assemblies in a VAV system may provide a fan-powered source of heat in addition to the HVAC supplying heated air to the zone via its zone damper. This type of design also can be used in the perimeter areas of a large building that might require heat on cold days independently of or in addition to that supplied by the operation of the central HVAC unit providing conditioned air to other parts of the building.
The use of an air flow sensor allows for control of a damper to limit the maximum air flow to a conditioned space independently of system pressure variations which may exist in large duct systems. In smaller systems, where pressure variations in the system are small, fan control can be used.
While the use of fan-powered mixing box assemblies combine the flexibility of decentralized heating with the benefits of VAV cooling systems, conventional systems have several disadvantages. First, because each fan-powered mixing box requires electric power for both the fan and heater, the large number of electrical connections can be expensive and difficult to install and maintain. Second, the high cost of using many smaller mixing box assemblies often requires the use of fewer, larger units serving larger zones resulting in a system with poor characteristics regarding the heating and cooling demands. Finally, the distributive nature of the system requires access to tenant areas in multiple occupancy buildings which is intrusive and can require extensive scheduling problems.
In the present invention, each of several large fan-powered mixing box assemblies can be treated as though it were a separate HVAC unit operated by a microcomputer controlled monitor with zone dampers controlled via VVT monitor-stats as described in the related U.S. applications identified above. None of the prior art fan-powered mixing box systems provide the ease of operation and maintenance and the degree of zone control as the fan-powered VVT system in accord with the present invention.